The present invention relates generally to ellipsometry. More particularly, the present invention pertains to ellipsometric methods and apparatus using radial symmetry.
Ellipsometry is an optical technique that uses polarized light to probe the properties of a sample. The most common application of ellipsometry is the analysis of very thin films. Through the analysis of the state of polarization of the light that interacts with the sample, ellipsometry can yield information about such films. For example, depending on what is already known about the sample, the technique can probe a range of properties including the layer thickness, morphology, or chemical composition.
Generally, optical ellipsometry can be defined as the measurement of the state of polarized light waves. An ellipsometer measures the changes in the polarization state of light when it interacts with a sample. The most common ellipsometer configuration is a reflection ellipsometer, although transmission ellipsometers are sometime used. If linearly polarized light of a knownorientationis reflected or transmitted at oblique incidence from a sample surface resultant light becomes elliptically polarized. The shape and orientation of the ellipse depend on the angle of incidence, the direction of the polarization of the incident light, the wavelength of the incident light, and the Fresnel properties of the surface. The polarization of the light is measured for use in determining characteristics of the sample. For example, in one conventional null ellipsometer, the polarization of the reflected light can be measured with a quarter-wave plate followed by an analyzer. The orientation of the quarter-wave plate and the analyzer are varied until no light passes though the analyzer, i.e., a null is attained. From these orientations and the direction of polarization of the incident light, a description of the state of polarization of the light reflected from the surface can be calculated and sample properties deduced.
Two characteristics of ellipsometry make its use particularly attractive. First, it is a nondestructive technique, such that it is suitable for in situ observation. Second, the technique is extremely sensitive. For example, it can measure small changes of a film down to sub-monolayer of atoms or molecules. For these reasons, ellipsometry has been used in physics, chemistry, materials science, biology, metallurgical engineering, biomedical engineering, etc.
As mentioned above, one important application of ellipsometry is to study thin films, e.g., in the fabrication of integrated circuits. In the context of ellipsometry, a thin film is one that ranges from essentially zero thickness to several thousand Angstroms, although this range can be extended in many cases. The sensitivity of an ellipsometer is such that a change in film thickness of a few Angstroms can usually be detected. From the measurement of changes in the polarization state of light when it is reflected from a sample, an ellipsometer can measure the refractive index and the thickness of thin films, e.g., semi-transparent thin films. The ellipsometer relies on the fact that the reflection at a material interface changes the polarization of the incident light according to the index of refraction of the interface materials. In addition, the polarization and overall phase of the incident light is changed depending on the refractive index of the film material as well as its thickness.
Generally, for example, a conventional reflection ellipsometer apparatus, such as shown in FIG. 1, includes a polarizer arm 12 and an analyzer arm 14. The polarizer arm 12 includes a light source 15 such as a laser (commonly a 632.8 nm helium/neon laser or a 650-850 nm semiconductor diode laser) and a polarizer 16 which provides a state of polarization for the incident light 18. The polarization of the incident light may vary from linearly polarized light to elliptically polarized light to circularly polarized light. The incident light 18 is reflected off the sample 10 or layer of interest and then analyzed with the analyzer arm 14 of the ellipsometer apparatus. The polarizer arm 12 of the ellipsometer apparatus produces the polarized light 18 and orients the incident light 18 at an angle 13 with respect to a sample plane 11 of the sample 10 to be analyzed, e.g., at some angle such as 20 degrees with respect to the sample plane 11 or 70 degrees with respect to the sample normal.
The reflected light 20 is examined by components of the analyzer arm 14, e.g., components that are also oriented at the same fixed angle with respect to the sample plane 11 of the sample 10. For example, the analyzer arm 14 may include a quarter wave plate 22, an analyzer 24 (e.g., a polarizer generally crossed with the polarizer 16 of the polarizer arm 12), and a detector 26. To measure the polarization of the reflected light 20, the operator may change the angle of one or more of the polarizer 16, analyzer 24, or quarter wave plate 22 until a minimal signal is detected. For example, the minimum signal is detected if the light 20 reflected by the sample 10 is linearly polarized, while the analyzer 24 is set so that only light with a polarization which is perpendicular to the incoming polarization is allowed to pass. The angle of the analyzer 24 is therefore related to the direction of polarization of the reflected light 20 if the minimum condition is satisfied. The instrument is xe2x80x9ctunedxe2x80x9d to this null (e.g., generally automatically under computer control), and the positions of the polarizer 16, the analyzer 24, and the incident angle 13 of the light relative to the sample plane 11 of the sample 10 are used to calculate the fundamental quantities of ellipsometry: the so called Psi, delta (xcexa8, xcex94) pair given by:             r      p              r      s        =      tan    ⁢          xe2x80x83        ⁢          Ψ      ⁢              (                  ⅇ                      j            ⁢                          xe2x80x83                        ⁢            Δ                          )            
where rp and rs are the complex Fresnel reflection coefficients for the transverse magnetic and transverse electrical waves of the polarized light, respectively. Form the ellipsometry pair (xcexa8, xcex94), the film thickness (t) and index of refraction (n) can be determined. It will be recognized that various ways of analyzing the reflected light may be possible. For example, one alternative is to vary the angle of the quarter wave plate and analyzer to collect polarization information.
Advances in microelectronics fabrication are rapidly surpassing current capabilities in metrology. In order to enable future generations of microelectronics, advanced specific metrology capabilities must be developed. Key among these metrology capabilities is the ability to measure the properties of complex layers of extremely thin films over sub-micron lateral dimensions.
Currently available ellipsometric techniques that measure material properties generally measure them over a large area. In other words, polarization measurements have been traditionally used to determine the thickness and refractive index of homogeneous films over a relatively large area. However, generally, determining the thickness and refractive index of homogeneous films over a relatively large area is inadequate for exceedingly small featured structures. For example, since the polarization state is effected significantly by diffraction from sub-micron features, the shape of such sub-micron features, e.g., critical dimensions of lateral or traverse structures such as gate dielectrics for transistor structures, is difficult to measure using current ellipsometric techniques that determine thickness and refractive index over relatively large areas. For example, the smallest spot that a conventional ellipsometer can measure is generally determined by the beam size, usually on the order of hundreds of microns. This essentially limits the application of conventional ellipsometers to samples with large and uniform interface characteristics.
The present invention exploits the polarization properties of high numerical aperture lenses to provide a novel ellipsometer, e.g., a micro-ellipsometer or spot ellipsometer. Unlike currently available techniques that measure material properties over a large area, the proposed ellipsometric method and apparatus can produce an ultra-high resolution image of material parameters by scanning a relatively small spot, e.g., sub-micron spot such as a spot smaller than 1 xcexcm. The proposed technique and apparatus results in accurate polarization measurements of exceedingly small features, providing new measurement capabilities.
Generally, an ellipsometer apparatus according to the present invention is a spot ellipsometer apparatus that uses radial symmetry to attain advantages over conventional techniques. For example, circularly polarized light may be focused to a spot on a sample using an objective lens and reflected therefrom. A radially symmetric ellipsometric signal representative of reflected light may be attained using a radially symmetric analyzer apparatus, e.g., two half-wave plates to produce a pure polarization rotation and a birefringent lens as a radially symmetric analyzer.
An ellipsometry method according to the present invention includes providing radially symmetric polarized light, e.g., radially polarized light or circularly polarized light, incident normal to a sample plane of a sample material. The radially symmetric polarized light is focused to a spot. The sample material reflects at least a portion of the focused radially symmetric polarized light as radially symmetric elliptically polarized light. The radially symmetric elliptically polarized light is operated upon to generate a radially symmetric ellipsometric signal representative of at least one characteristic of the sample material. The radially symmetric ellipsometric signal is detected for use in determining the at least one characteristic of the sample material.
In one embodiment of the method, the method includes operating on the radially symmetric elliptically polarized light to generate a radially symmetric ellipsometric signal using a radially symmetric analyzer. For example, the radially symmetric analyzer may be a birefringent lens, a Brewster angle reflector, or a circular metallic grating.
In another embodiment of the method, a pure polarization rotator may be used in the generation of the radially symmetric ellipsometric signal. For example, the pure polarization rotator may be two-half wave plates, a Faraday rotator, or a rotator including a first quarter wave plate, a variable retarder, and a second quarter wave plate.
In another embodiment of the method, the radially symmetric polarized light is focused to a spot on the sample material using an objective lens. The sample material reflects at least a portion of the focused radially symmetric polarized light as radially symmetric elliptically polarized light. In an alternate embodiment, a solid immersion lens having a lower surface is provided adjacent the sample material. The radially symmetric polarized light is focused to a spot on the lower surface of the solid immersion lens using an objective lens. The sample material reflects at least a portion of the focused radially symmetric polarized light as radially symmetric elliptically polarized light.
An ellipsometer apparatus according to the present invention includes an illumination source operable to provide radially symmetric polarized light, e.g., circularly polarized light or radially polarized light, incident normal to a sample plane of a sample material. An objective lens focuses the radially symmetric polarized light to a spot and collects reflected light from the sample material illuminated using the spot. A radially symmetric analyzer apparatus is adapted to receive the reflected light from the objective lens and provide a focused radially symmetric ellipsometric signal based on the reflected light representative of a characteristic of the sample material. A detector is operable to detect the focused radially symmetric ellipsometric signal for use in determining the at least one characteristic of the sample material.
In various embodiments of the apparatus, the radially symmetric analyzer apparatus may include a pure polarization rotator adapted to receive the reflected light and provide rotated reflected light. Further, the analyzer may include a birefringent lens adapted to focus the rotated reflected light onto the detector, a Brewster angle reflector, or a circular metallic grating.
In another embodiment of the apparatus, the radially symmetric analyzer apparatus includes two half wave plates adapted to receive the reflected light and provide rotated reflected light, a Faraday rotator comprising a Faraday effect material responsive to an applied current adapted to receive the reflected light and provide a rotated reflected light, or pure polarization rotator including a first quarter wave plate, a variable retarder responsive to an applied voltage, and a second quarter wave plate adapted to receive the reflected light and to provide rotated reflected light. Further, the analyzer apparatus may include a radially symmetric analyzer.
In another embodiment, the objective lens is adapted to focus the radially symmetric polarized light to a spot on the sample material. In an alternate embodiment, the apparatus further includes a solid immersion lens having a lower surface positioned adjacent the sample material. The objective lens focuses the radially symmetric polarized light to a spot on the lower surface of the solid immersion lens, e.g., a semi-spherical solid immersion lens.
In another embodiment of the apparatus, the apparatus further includes a first beam splitter for passing the radially symmetric polarized light incident normal to the sample plane and incident on the objective lens. Further, the beam splitter diverts the reflected light collected by the objective lens. A second beam splitter is optically coupled to the first beam splitter to pass the diverted reflected light to the radially symmetric analyzer apparatus. The second beam splitter is adapted to compensate for polarization distortion of the incident radially symmetric polarized light passed by the first beam splitter.
In yet another ellipsometer apparatus according to the present invention, a nulling ellipsometer using radial symmetry is described. In one embodiment of the nulling ellipsometer, the analyzer apparatus of the ellipsometer includes a fixed quarter wave plate, a rotating analyzer, a lens, and a detector, e.g., a charge coupled device camera.